1. Field of the Invention
The present invention relates to an imaging apparatus capable of capturing high-definition color still images by means of a plurality of solid-state imaging devices.
2. Description of the Related Art
Recently, imaging apparatuses for still image, generally referred to as "electronic still camera", have been commercialized as an imaging apparatus for picking up still images. On the other hand, home video cameras are in practical usage today as an imaging apparatus for picking up motion images. These imaging apparatuses typically employ a solid-state imaging device as means for picking up image light from an object. The solid-state imaging device is a so-called two-dimensional CCD (Charge Coupled Device) image sensor. The solid-state imaging device has a plurality of light-receiving regions arranged in the form of matrix on an imaging plane which is a two-dimensional plane. When the image light from the object is picked up, the imaging apparatus causes the image light from the object to be focused on the imaging plane of the solid-state imaging device so that the light is received by each light-receiving region. The image light is photoelectrically converted into an electric signal indicative of an amount of light received by the light-receiving region and thereafter, recorded in a recording medium as an image signal. If such an image signal is discretely committed to display on a video display device, a still image is produced. If the image signals are sequentially committed to display in the order of signal acquisition, a motion image is produced.
The image picked up by such an imaging apparatus is composed of a plurality of pixels corresponding to light-receiving regions of the solid-state imaging device in arrangement and number. That is, each solid-state imaging device samples the luminance of image light, which changes continuously over the space, at a spatial sampling frequency. The spatial sampling frequency is reciprocal of an array period of the pixels. This configuration allows the changes in the luminance of image light to be smoothed on a pixel basis. Hence, a greater number of pixels leads to a higher resolution of the resultant image.
A consumer solid-state imaging device employed by the prior-art imaging apparatuses for still and motion images typically includes about 400,000 light-receiving regions. The consumer solid-state imaging device conforms to the specifications for apparatuses for displaying a motion image of NTSC (National Television System Committee) standard. Accordingly, the prior-art imaging apparatus for still image outputs an image signal in the same signal format with an image signal of motion image. Resolution of a still image composed of 400,000 pixels is sufficient for a motion image of NTSC system. Accordingly, when a signal representative of an still image obtained by the imaging apparatus for still images is committed to display on a video display device of NTSC standard, the resultant image substantially satisfies the viewer.
Unfortunately, the consumer solid-state imaging device has a smaller number of light-receiving regions than that required by the standards for so-called "Hi-Vision" devices adapted to pick up high definition images. Accordingly, when a signal representative of a still image obtained by the conventional imaging apparatus is committed to display on the high-definition video display device, the resultant image has a lower quality than desired. Furthermore, the resolution of a still image comprising about 400,000 pixels is far lower than the resolution of an image picked up by a still camera based on the silver halide film of the prior art. For these reasons, high quality of still images has been called for also in the case of electronic still camera.
As a technique of improving the resolution of image, there has been proposed a technique of increasing the number of pixels constituting the image. That is, the number of light-receiving regions included in a solid-state imaging device of the same size is increased. For example, a Hi-Vision solid-state imaging device includes approximately two million light-receiving regions, which outnumber the light-receiving regions of a consumer solid-state imaging device of NTSC standard. Thus, when still images are obtained by the imaging operation with such a Hi-Vision solid state imaging device, resolutions of the still images obtained are increased as compared to the case of using the consumer solid-state imaging device. However, the Hi-Vision solid-state imaging device has more precise configuration than the consumer solid-state imaging device and hence, the availability thereof is limited.
There has been proposed an imaging operation utilizing an image shifting technique as an alternative solution to improvement of the resolution of the still image obtained by the consumer solid-state imaging device. The image shifting is a technique for shifting an image light receiving position in the solid-state imaging device. In the imaging operation utilizing image shifting, the light-receiving position of the image light from an object within the solid-state imaging device is changed plural times by image shifting, and image light is imaged in each change. Then a plurality of images obtained by the imaging operation are superimposed on one another so that the light-receiving positions of the images correspond to one another, to generate output images.
The solid-state imaging apparatus employing an image shift mechanism has been disclosed in Japanese Unexamined Patent Publication JP-A 63(1988)-284980. The Publication discloses a solid-state imaging apparatus wherein a planeparallel plate light transmitting is interposed between a collective lens for collecting light from an object for image shifting and a solid-state imaging device. The planeparallel plate assumes a first state to be positioned vertical relative to the optical path and a second state to be inclined on a diagonal axis forming an angle of 45.degree. with the horizontal and vertical directions of the field of view. The solid-state imaging device picks up image light to obtain a first original image when the planeparallel plate is in the first state, and then picks up image light to obtain a second original image when the planeparallel plate is inclined to be in the second state. The two original images thus acquired are superimposed on each other for generating an output image.
FIG. 25 is a schematic diagram illustrating an equivalent pixel array to that of an output image, which is a color image. The light-receiving regions of this solid-state imaging device are arranged in a matrix at horizontal array period PH and vertical array period PV. In this case, the produced output image has pixels arranged in an array at the horizontal array period of (PH/2) and the vertical array period of (PV/2), accomplishing four-fold increase in the number of pixels as a whole. In FIG. 25, a pixel s1 denotes a real pixel acquiring pixel data from the first original image picked up in the first state. A pixel s2 denotes a real pixel acquiring pixel data from the second original image obtained in the second state. FIG. 25 shows the real pixels by way of diagonal shading. That is, the real pixels acquiring the respective pixel data are arranged in a checkerboard pattern in the output image. A virtual pixel with no pixel data adjoins two real pixels along the respective array directions. Accordingly, these virtual pixels may each obtain pixel data by way of interpolation of an average value of the pixel data of, for example, the four adjacent real pixels. In this manner, this prior-art solid-state imaging apparatus produces a high-resolution image composed of pixels four times as many as the light-receiving regions of the solid-state imaging device.
As described above, the imaging device for motion images and the imaging device for still images have similar constructions. Accordingly such an imaging device is desired which is capable of obtaining both images for motion image and still image, in which the imaging devices for motion images and still images are combined. When the imaging operation including the image shift processing is conducted to obtain images for a motion image, image signals for images for one motion image are generated from two original images successively obtained by the solid-state imaging device. Consequently in this case an equivalent exposure time of the solid-state imaging device required for obtaining a single image is doubled or more as compared to a device without image shifting. However, since it is necessary to continuously obtain images for the motion image at predetermined time intervals, the equivalent exposure time of the solid-state imaging device required for obtaining the images for the motion image must be shorter than that required for obtaining an image for a single still image.
Furthermore, since the solid-state imaging device typically used corresponds to a signal format for motion images in array and number of the light-receiving regions, a signal format of image signals of an original image directly outputted from the solid-state imaging device conforms to the signal format for motion images. However, since an output image obtained by the imaging operation including image shifting has a larger number of pixels or pixel-rows and -columns than the original image, the image signal of the output image does not conforms to the signal format for motion images. Accordingly when images for motion images are imaged by the imaging operation including image shifting, a format of the acquired image signal of the output image must be transformed into a format for the motion image. This leads to a need for further increasing the signal processing speed.
The solid-state imaging device images by focusing original image light on a finite number of light-receiving regions. Accordingly, a continuous change of the original image in luminance on the spatial axis is sampled at a sampling frequency which is an inverse number of an array period in a direction parallel to the axis of the light-receiving region. Thereby, the image signal of the original image becomes a digital data signal and contains therein a reflected component. When interference occurs between an image of an object and the arrangement of the light-receiving regions of the solid-state imaging device, the reflected component is superimposed on a component desired in the signal processing with the result that moire is caused in the output image.
In order to prevent the moire from being caused, an optical system of the solid-state imaging apparatus often includes an optical filter for exclusively transmitting a predetermined spatial frequency component of light. The wave-filtering band of this filter is set such that high frequency components of the spatial frequencies in a range where reflected components of the image signal are generated, in the image light imaged by the solid-state imaging device, be attenuated. The output image obtained through the imaging operation including the image shift processing has a pixel array period of half the pixel array period of the original image. Thus, in the output image produced through the imaging operation including the image shift processing, an equivalent sampling frequency for producing this image is at a level twofold over the sampling frequency of this imaging device. That is, the sampling frequency for the original image is half the sampling frequency for the output image produced through the imaging operation including the image shift processing. The more the high frequency components contained in the spatial frequency components of image light to be picked up, the higher the resolution of the resultant image. Therefore, an upper limit of the wave-filtering band of the optical filter is often set to a sampling frequency of the image shift processing. When both an image for motion image and an image for still image are picked up by the imaging apparatus, it is possible only in the case of picking up the image for motion image for the imaging apparatus to output an image signal of an original image directly as an image signal of an output image without image shifting. This apparatus uses the same optical system when performing the image shift processing as well as when not performing the image shift processing. When such an optical filter having the above-mentioned filtering features is interposed within the optical system, an occurrence of the moire fringe during the image shift processing can be prevented. However, when the original image is directly used as the output image, the reflected components remain, thus the moire fringe is caused in the motion image. Further, to the contrary, when the upper limit of the wave-filtering band is set to the sampling frequency for the original image, the high frequency components of the spatial frequency of the output image are excessively attenuated during the image shift processing, which results in decrease in resolution of the output image.
There is known an imaging optical system disclosed in a Japanese Unexamined Patent Publication JP-A 4-236585 (1992) as a prior art relating to the above-mentioned low-pass filter. The imaging optical system comprises the low-pass filter in order to avoid the moire caused by the interference between an object image and the solid-state imaging device. The spatial frequency restriction effect is variable, and when a distance between an object to be imaged and an objective lens exceeds a distance range in which focusing of the objective lens can be conducted, the spatial frequency restriction effect is reduced in order that the resolution of the object image is excessively reduced. In a apparatus comprising the prior art optical system, an image to be obtained is either an image for a still image or an image for a motion image, and switching of image is not conducted. Accordingly output images are the same in number of pixel and arrangement, and change in spatial frequency due to these causes is not considered. Accordingly it is difficult to avoid the moire due to a cause regarding pixel number and arrangement, even if the spatial frequency restriction effect is changed under the above-mentioned judging conditions.